Carbon Honeycombs Research Articles

Selective uptake and desorption of carbon dioxide in carbon honeycombs of different sizes

Carbon honeycombs (CHs) are new carbon cellular structures, very promising in many respects, in particular, for high-capacity storage of various materials, especially in gaseous and liquid forms. In this study, we report a strong uptake of carbon dioxide kept inside carbon honeycomb matrices up to temperatures about three times higher as compared with CO2 desorption at ≈ 90 K from flat solid surfaces in vacuum where we conduct our high-energy electron diffraction experiments. Desorption of CO2 from CH matrices upon heating exhibits non-monotone behavior, which is ascribed to carbon dioxide release from CH channels of different sizes. It is shown that modeling of CO2 uptake, storage, and redistribution in the thin CH channels of certain types and orientations upon heating can explain experimental observations.

Wood frame structured carbons with integrated sieving layer for propylene/propane separation

Adsorption separation of propylene/propane is of great importance in producing the key feedstock of propylene. The core challenge lies in the design of high-performance adsorbent materials capable of high adsorption uptake and selectivity. Here we report a proof-of-concept adsorbent of wood frame structured carbons with an integrated sieving layer, which is prepared on the basis of natural woods by imparting functional polymer layers in the wood cells. The polymer layers in the wood frame penetrate into the wood walls and transform into integrated sieving layer in the resultant carbon honeycombs, rather than block the wood channels. The effective sieving layer as well as the ordered free transport channels enable this type of material to display excellent propylene/propane separation performance, showing a high propylene adsorption uptake of 1.72 mmol g−1, and an ideal adsorbed solution theory and dynamic selectivity up to 3.8 × 104 and 139 at 298 K and 1 bar, respectively. This work provides a unique idea for designing structured carbon adsorbents with selective sieving layer, which may apply to other molecular recognition, separation, and purification processes beyond propylene/propane separation.

Tensile and Compressive Behavior of CHC‐Reinforced Copper using Molecular Dynamics

Graphene has been extensively studied as nanofiller to produce ultra‐strong and ductile metal nanocomposites but challenges such as poor adhesion at the metal–carbon interface have yet to be met. Carbon honeycombs (CHCs) are highly porous 3D graphene networks that possess a very large surface area‐to‐volume ratio, an outstanding physical absorption capacity and notable mechanical properties. Herein, these recently synthetized 3D CHCs are introduced in copper as nanofillers, and the mechanical properties of the nanocomposites, such as elastic modulus, tensile strength, failure strain, compressive strength, and critical strain, are obtained using molecular dynamics simulations. Three CHC lattice types are studied, and the metal–carbon interface is accurately modeled by using melting and recrystallization of the copper matrix around the nanofiller. Gains between 28% and 50% are obtained for the Young's modulus, while the tensile strength improved between 43% and 49%. Pullout tests reveal that the copper nanopillars that form by the filling of the honeycomb cells of CHC by copper atoms considerably increase the pullout force and are responsible for improvements in adhesion and in loading stress transfer.

Structure and Raman Spectra of Exotic Carbon Microcrystals from Meteoritic Dust of Chelyabinsk Superbolide.

The Chelyabinsk superbolide, the largest in XXI century, which exploded on 15 February 2013 over snowy fields of Southern Urals was a historic event not just only because of its massive scale and explosive power. High-temperature, high-pressure conditions in the front shock wave caused intense ablation of the asteroid material and formation of huge amount of meteoritic dust dispersed in the atmosphere during the flyby. Massive snowfalls just few days before and after the event conserved precipitated meteoritic dust in thin layer of snow which was collected and studied later. The most intriguing and challenging material discovered in the dust was closed-shell 10-70 µm exotic polygonal graphitic carbon microcrystals of undisclosed nature. Using optical and electron microscopy and Raman spectroscopy the atomic structure of closed-shell microcrystals was thoroughly studied and their graphitic nature was revealed. It was found that some of the particles formed by multilayer graphitic polygonal shells have extensive hollows inside. Comparative microscopic and spectroscopic analysis of meteorite exotic carbon microcrystals with different graphite species, carbon onions, and diamond revealed two distinctively different closed-shell carbon particles. The first type of the particles can be attributed to carbon onions with characteristic graphite nanocrystalline basic structural units (BSU) of 49 nm lateral size and less and, probably, BSU heteroatomic termination necklace with oxygen content comparable to 1.1% and more. It was shown that the second type of unique graphitic carbon particles of a convex shape and perfect hexagonal symmetry with lateral dimensions of 14 µm correspond to multiply twinned closed-shell graphite microcrystals with polyhexacyclooctadecane (-C18-)n core wrapped by multiple layers of carbon honeycombs with low (<1%) content of oxygen termination necklace.

Modulating thermal transport in a porous carbon honeycomb using cutting and deformation techniques.

During the past few years, there has been a flurry of investigations on the lattice thermal transport of three-dimensional (3D) graphene, however, few studies have detailed how to adjust this property effectively using the presently available engineering technologies. In this work, the thermal transport properties of a porous single layer carbon honeycomb (SL-dCHC-2) and its mechanical response are systematically studied. We show that the thermal conductivity of SL-dCHC-2 can be adjusted effectively by varying the tensile strain, and its value is enhanced by up to 11.3 times with 8% strain as compared to the unstrained case. This value is significantly larger than what was observed for other two-dimensional (2D) materials such as silicene (∼7 times larger). This outstanding behavior is explained by the phonon mode level, indicating that a profound increase of the thermal conductivity under tensile strain is attributed to the enhancement of the phonon lifetime. In addition, the trend for the root mean squared displacement, which is closely related to the phonon anharmonic effect, correlates with the non-monotonic response of the dimerized C-C bonds at the linkage of the structure. These investigations and obtained results provide important guidance to develop 3D carbon honeycombs for several different purposes, such as for use as molecular sieves and in water purification applications.

H2S catalytic removal at low temperature over Cu- and Mg- activated carbon honeycombs

A series of monometallic and bimetallic Cu/Mg oxide-based structured catalytic sorbents were synthesized by impregnation of extruded activated carbon honeycombs and tested for the purification of simulated biogas from H2S at room temperature. Dynamic H2S (100ppmv) removal tests were run at short contact time under moist conditions in the presence of O2 to compare sulfur capture capacities and to investigate kinetic aspects of the oxidation reactions involved in the mechanism at varying the composition of the active phase. Fresh, exhausted and thermally regenerated honeycomb catalytic sorbents were characterized by N2-adsorption, PSD, XPS, TG-MS to shed light on the specific nature of S-species formed upon catalytic oxidation, the maximum capacity and the efficacy of a simple thermal regeneration treatment at 600 °C. Under moist conditions, the surface chemistry resulted dominated by the basic nature of MgO, and bimetallic CuMg catalytic sorbents show fast H2S removal rates and large S-capacity coupled to easy and complete regenerability.

Prediction and optimization of the thermal transport in hybrid carbon-boron nitride honeycombs using machine learning

The recently discovered carbon honeycombs (CHCs) and boron nitride honeycombs (BNHCs) are found to have the similar molecular structures but different thermal properties. Thus, through appropriately patching together CHCs and BNHCs, the hybrid carbon-boron nitride honeycombs (C–BNHCs) with tunable thermal conductivity can be achieved. In this paper, the machine learning (ML) method together with molecular dynamics simulations is employed to study the thermal transport property of C–BNHCs, and also utilized to design the structures of C–BNHCs for the specific thermal conductivity. Our forward learning study reveals a big difference in the thermal conductivities of C–BNHCs with the same BNHC doping level but different doping arrangements. Meanwhile, a nonmonotonic relation is observed between the thermal conductivity of C–BNHCs and their doping concentration, which, according to our analyses of the phonon density of states and spectral thermal conductivity, is attributed to the complicated phonon scattering behaviors in C–BNHCs. In addition, our ML-based method exhibits the high accuracy and efficiency in the inverse design of C–BNHCs with any specific thermal conductivity. Moreover, as for a target thermal conductivity, the present ML-based inverse design method can output numerous potential optimal structures at once for C–BNHCs, which will greatly shorten the design period of C–BNHCs.

Auxetic Carbon Honeycomb: Strain-Tunable Phase Transitions and Novel Negative Poisson's Ratio.

Auxetic structure and tunable phase transitions are fascinating properties for future application. Herein, we propose two three-dimensional (3D) carbon honeycombs (CHC), known as Cmcm-CHC and Cmmm-CHC. Based on first-principles calculations, these novel 3D materials exhibit auxeticity with a fascinating negative Poisson’s ratio, which stems from (i) the puckered structure of Cmcm-CHC along the tube axis and (ii) significant change of angle-dominant deformation for Cmmm-CHC in the armchair direction. In addition, the moderate strain drives semimetal to semiconductor phase transition in CHCs, which thoroughly establishes its C–C bond formation. In the meantime, two new phases, namely P63/mmc-CHC and P6/mmm-CHC, form and exhibit semiconductor characteristics. Our results also show that Cmcm-CHC and P63/mmc-CHC are superhard materials. The outstanding negative Poisson’s ratio and phase transition properties make CHCs highly versatile for innovative applications in microelectromechanical and nanoelectronic devices.

Theoretical analysis of high strength and anti-buckling of three-dimensional carbon honeycombs under shear loading

Three-dimensional carbon honeycombs (3D C-honeycombs) are considered as a promising matrix material for new composites, while their mechanical behaviors under complex stress status remain unclear. Here, shear response of 3D C-honeycombs is systematically investigated with molecular dynamics (MD) simulations and mechanical analysis. Two deformation modes are observed for 3D C-honeycombs under in-plane and out-of-plane shear loading. A mechanical model is proposed to describe the deformation of 3D C-honeycombs under different shear loading, and agrees well with the MD simulations. Under in-plane shear loading, the initial elastic deformation is dominated by distortion of hexagon structures, shear modulus increases as increasing shear strain, and the tension of side walls leads to fracture. Under out-of-plane shear loading, the structure of 3D C-honeycombs can remain stable until fracture regardless of side wall buckling, and the tensile stress concentration at junctions gives rise to fracture. For both the two modes, the junction type has a negligible effect on shear strength of 3D C-honeycombs. The stability of junctions plays a dominant role in the shear response of 3D C-honeycombs, and thus the superb mechanical properties of graphene can be largely inherited by the 3D C-honeycombs.

Temperature dependent mechanical properties of graphene based carbon honeycombs under tension and compression

Graphene-based carbon honeycombs (GCHs) show outstanding physical and mechanical properties, and excellent potential applications. Based on molecular dynamic simulations, we find that temperature plays an important role in the mechanical properties of GCHs. In both the in-plane and out-of-plane cases, the tensile strength and fracture strain of GCHs decrease significantly as temperature increases from 10 to 1800 K. Under compressive deformation, the in-plane compressive stress-strain curves vary slightly when the temperature is lower than 900 K, and change acutely at temperature higher than 1200 K. The out-of-plane compressive failure stress and strain decrease prominently with increasing of temperature from 10 to 1800 K. Our results provide a theoretical guidance for GCHs application in different temperature environment.

Effects of cell defects on the mechanical and thermal properties of carbon honeycombs

Cell defects are inevitable during the fabrication of carbon honeycombs (CHCs), which, however, were tacitly ignored in previous studies. In this work, the effects of defects including Stone-Wales (SW) defect and single-wall vacancy (SV) defect on the mechanical and thermal properties of CHCs are investigated by using molecular dynamics simulations. Our results show that the Young’s modulus of CHCs has nearly a constant value around 16.5 GPa, which is almost independent with the defect concentration. However, the tensile strength and fracture strain are reduced by about 40% and 20%, respectively, when the CHCs have a defect concentration of 2%. Additionally, the influence of SV defect on the fracture properties of CHCs is generally larger than that of its SW counterpart. The influence of cell defects on the mechanical properties of CHCs is well explained by the theories of damage and fracture mechanics. Our simulation results also show a reduction in the thermal conductivity of CHCs containing SW and SV defects. This reduction can be up to 23% when the defect concentration of CHCs is 2%. The reduced thermal conductivity observed in defective CHCs can be attributed to the wavy configuration of the component graphene nanoribbons in defective cells, which can increase the phonon scattering of CHCs. In addition, the reduced heat capacity of CHCs with SV defects can be another important factor inducing the reduction in their thermal conductivity.

Thermal transport in three-dimensional carbon honeycombs

Three-dimensional carbon honeycombs were experimentally discovered recently. In this work, we study the thermal conductivity of zigzag carbon foams with different sidewall widths. We found the anisotropic thermal conductivity along different directions. We noted that the local buckling introduced by the interconnections severely impedes the phonon transport. The phonon density of states was analyzed to compare the phonon modes in the planar and the interconnection regions. The structure-thermal transport property relationship obtained in this study provides important insights for future applications of three-dimensional carbon honeycombs, such as energy storage, catalysis, microelectronics, and the like.

Absorption-desorption of carbon dioxide in carbon honeycombs at elevated temperatures

The recently synthesized honeycomb carbon allotrope has numerous potential applications, in particular for storage of gases inside carbon matrices. In this work this carbon form was experimentally studied in its denser form in order to estimate the upper temperature limit for keeping a gas inside the cellular structure. Along with the previously reported random honeycombs of a zigzag type we have also revealed the densest armchair structure. The mechanism of absorption-desorption of carbon dioxide studied by means of high energy electron diffraction at low temperatures showed the two — stage character of the observed desorption at elevated temperatures. This effect is associated to the weaker or stronger bonding of molecules with pore walls depending on the specific configuration of channels with different sizes. We have found that complete desorption of CO2 does not occur even at the temperatures about three times higher as compared with the sublimation point of carbon dioxide in our vacuum conditions.

Atomistic Study of Mechanical Behaviors of Carbon Honeycombs.

With an ultralarge surface-to-volume ratio, a recently synthesized three-dimensional graphene structure, namely, carbon honeycomb, promises important engineering applications. Herein, we have investigated, via molecular dynamics simulations, its mechanical properties, which are inevitable for its integrity and desirable for any feasible implementations. The uniaxial tension and nanoindentation behaviors are numerically examined. Stress–strain curves manifest a transformation of covalent bonds of hinge atoms when they are stretched in the channel direction. The load–displacement curve in nanoindentation simulation implies the hardness and Young’s modulus to be 50.9 GPa and 461±9 GPa, respectively. Our results might be useful for material and device design for carbon honeycomb-based systems.

Electronic band structure of carbon honeycombs

In this article, we present the electronic properties of various carbon honeycombs (C-honeycombs) with different cell sizes and junction types based on ab initio calculations. Among the three typical C-honeycombs with junctions connected via the zigzag edge, the armchair edge, or the hybrid edges of graphene nanoribbons, it is found that the zigzag C-honeycombs and the hybrid ones exhibit metallic properties. However, the armchair C-honeycombs are metallic only if the number of atomic planes N of the graphene nanoribbon in the C-honeycomb sidewall follows N = 3p+1, with p as an integer and are semiconducting for other N. For those semiconducting ones, the bandgap of the C-honeycomb is about 1.19 eV with the narrowest sidewall (5.2 Å) and monotonically decreases to about 0.15 eV when the width is increased to 34.8 Å. The semiconducting nature of such C-honeycombs with nanometer size pores may be used for lightweight semiconductor in electronic devices or supercapacitor energy storage. As a comparison, we show that all boron honeycombs are metallic, and their electronic properties are independent of sidewall widths.

CO2 capture from ambient air using hydrated Na2CO3 supported on activated carbon honeycombs with application to CO2 enrichment in greenhouses

CO2 capture from ambient air is an interesting option for CO2 enrichment in greenhouses. In this study, adsorbents comprised of hydrated Na2CO3 supported over activated carbon honeycombs were prepared, characterized and tested for CO2 capture from air. The adsorption of H2O and the formation of the hydrates were studied by means of FT-IR spectroscopy. The inlet CO2 concentration showed to have a major influence on the conversion yield into NaHCO3, and the results fitted well to the Toth model. A statistical model of the CO2 capture capacity was obtained to get insight into the key parameters of the adsorption process. The air temperature and its moisture content showed to have the largest impact on the CO2 capture, while the flow rate had a minor influence. The chemical reaction path during the CO2 adsorption showed to be determined by the relative humidity conditions inside the reactor. Addition of more salt on the carrier showed to improve the CO2 capture capacity, but this is limited by the strength of the honeycomb carrier. Finally, a preliminary desorption test via a mild temperature and moisture swing was run to assess the feasibility of the process for application in greenhouses. The results showed that the required volume of adsorbent would be roughly 1/1000 of the total volume of a closed greenhouse assuming a target CO2 level of 1200 ppm.

Nexus networks in carbon honeycombs

Nexus metals represent a new type of topological material in which nodal lines merge at nexus points. Here, we propose novel networks in nexus systems through intertwining between nexus fermions and additional nodal lines. These nexus networks can be realized in several recently synthesized carbon honeycomb materials. In these carbon honeycombs, we demonstrate a phase transition between a nexus network and a system with triply-degenerate points and additional nodal lines. The Landau level spectra show unusual magnetic transport properties in the nexus networks. Our results pave the way toward realizations of new topological materials with novel transport properties beyond standard Weyl/Dirac semimetals.

A nonlocal continuum model for the buckling of carbon honeycombs

Using molecular dynamics (MD) simulations and Eringen’s nonlocal elasticity theory, in this paper we comprehensively study the small-scale effects on the buckling behaviours of carbon honeycombs (CHCs). The MD simulation results show that the small-scale effects stemming from the long-range van der Waals interaction between carbon atoms can significantly affect the buckling behaviours of CHCs. To incorporate the small-scale effects into the theoretical analysis of the buckling of CHCs, we develop a nonlocal continuum mechanics (CM) model by employing Eringen’s nonlocal elasticity theory. Our nonlocal CM model is found to fit MD simulations well by setting the nonlocal parameter in the nonlocal CM model as 0.67. It is shown in our MD-based nonlocal CM model that when the cell length of CHCs is smaller than 7.93 A the influence of small-scale effects on the bucking of CHCs becomes unnegligible and the small-scale effects can greatly reduce the critical buckling stress of CHCs. This reduction in critical buckling stress induced by the small-scale effects becomes more significant as the length of the cell wall decreases. Moreover, CHCs are found to display two different buckling modes when they are under different states of loading. The critical condition for the transition between these two buckling modes of CHCs can be greatly affected by the small-scale effects when the vertical cell wall and the inclined cell wall of CHCs have different lengths.

Inner edge magnetisms in carbon honeycombs

We show by comprehensive ab initio calculations that sp2 carbon honeycombs recently synthesised by Krainyukova et al. [Phys. Rev. Lett. 116, 055501 (2016)] exhibit antiferromagnetism, not only at the inner edge of the zigzag ribbon component but also at the chain of sp2 carbon that joins three zigzag ribbons. The π antiferromagnetism at the joint chain has spin orientations that alternatively change along the axis and stems from a super-exchange mechanism. Along with the spin-polarization, the joint chain conduction channel opens an energy gap. The spin-polarization of the zigzag edge due to the magnetic instability of the localized edge states is less stable in energy. Through hole doping, the zigzag edge antiferromagnetism is enhanced and stabilized as the magnetic ground state, along with the re-opening of the joint chain conduction channel. When the carbon honeycombs are reconstructed into sp3-sp2 hybrid honeycombs, the π states of the joint are diminished, but the zigzag edge magnetism is preserved. Our results propose carbon honeycombs as novel magnetic carbon with competing polarization configurations.

Anisotropic thermal conductivity in carbon honeycomb

Carbon honeycomb, a new kind of 3D carbon allotrope experimentally synthesized recently, has received much attention for its fascinating applications in electronic device and energy storage. In the present work, we perform equilibrium molecular dynamics (EMD) to study the thermal transport properties of carbon honeycombs with different chirality. It is found that the thermal conductivity along the honeycomb axis () is three times larger than that normal to the axis (), which shows strong anisotropy reflecting their geometric anisotropy. Lattice dynamics calculations reveal that this anisotropy stems from the orientation-dependent phonon group velocities. Moreover, when ambient temperature () increases from 200 K to 800 K, the dependence of is observed due to the enhanced Umklapp scattering. The detailed phonon spectra analyses indicate phonon group velocities are insensitive to the variation of ambient temperature, and the temperature dependence of the relaxation times of low-frequency phonons (<20 THz) follows behavior. Our results have a certain guiding significance to develop carbon honeycomb for effective thermal channeling devices.